1. Field of the Invention
The present invention relates to a sensor sheet capable of measuring the distribution of forces externally applied.
2. Description of Related Art
U.S. Pat. No. 4,734,034 and U.S. Pat. No. 4,856,993 disclose a sensor sheet in which a polyethylene terephthalate (PET) film on which vertical stripes are printed with pressure-sensitive resistive ink and another PET film on which horizontal stripes, that is, perpendicular to the vertical stripes, are printed with pressure-sensitive resistive ink are attached to each other so that the pressure-sensitive resistive ink stripes printed on the respective films intersect with each other. In this sensor sheet, a force externally applied changes the contact resistance between the two PET film at the portion to which the force has been applied. Therefore, by detecting changes in contact resistance along the respective pressure-sensitive resistive ink stripes, the distribution of externally applied forces, that is, pressures, can be measured.
However, the above-described sensor sheet can detect only the component of the externally applied force perpendicular to the sheet, but can not detect the other directional components. That is, even when forces are applied obliquely, that is, not perpendicularly, to the sensor sheet, the sheet can only measure the distribution of forces like a case in which forces corresponding to the components of the oblique forces perpendicular to the sheet are applied to the sheet. Thus, this sensor sheet can not perform detailed analysis of the distribution of forces externally applied.
For the above reason, the applicant of the present application has proposed in Japanese Patent Unexamined Publication No. 2004-117042 a sensor sheet capable of detecting different directional components of externally applied forces. This sensor sheet includes a number of sensor cells arranged in a matrix. Each sensor cell has electrodes corresponding to X-, Y-, and Z-axes. A force externally applied changes the contact resistance of pressure-sensitive resistive ink disposed between the electrodes corresponding to each axis. Thus, by detecting a change in the resistance value, the sensor sheet can detect the X-, Y-, and Z-axial components of the externally applied force.
The above-described sensor sheet can detect the X-, Y-, and Z-axial components of each force externally applied, however, it has various problems as follows. First, because each sensor cell has a number of electrodes corresponding to different axes, this makes it difficult to miniaturize the sensor sheet. In particular, when a number of sensor cells are disposed close to each other, this makes it very difficult to extend out lead wires from all electrodes. In addition, each sensor cell is complex in construction, and this causes very bad efficiency for increasing the density of the sensor cells. Further, when pressure detected by each sensor cell is converted into a voltage proportional to the magnitude of the pressure, an R/V converter is required to convert resistance into voltage in one-to-one proportion for any contact resistance. This makes the converting circuit large-scale.
The above-described problems apply to not only a sensor sheet in which forces are detected on the basis of changes in the contact resistance of pressure-sensitive resistive ink between electrodes, but also a sensor sheet in which forces are detected on the basis of changes in the capacitance of capacitors between electrodes.